The present invention relates to an information storage system adaptable to an optical disk storage system using a plurality of disks as storage mediums thereof and capable of performing data input and output operations by changing the disks, and having a mechanism for optimally locating data.
In recent years, a large-scale system has been structured in which a multiplicity of computers are connected in a network environment to interchange information. A system of the foregoing type employs a method in which a computer, called a server, uses an information storage device having a very large capacity to concentrically manage all information (images, data bases, audio information and characters) in the system. The above-mentioned system involves considerable enlargement of the quantity of information in the system which must be managed by the server. Therefore, there arises a requirement to enlarge the storage capacity of the information storage device.
The information storage device, as known, includes a hard disk unit (a HDD) and a magnetooptical disk unit. In particular, an information storage system has been developed which uses a multiplicity of magnetooptical disks (hereinafter simply called as "disks") as the storage mediums therein and which is called a magnetooptical disk auto-changer (also called a "jukebox") capable of changing and using the disks.
An information storage system of the foregoing type for changing and using the plural storage mediums uses a concept called a volume as a logical storage unit for managing information to be stored. In general, the volume, which is a physical storage unit, corresponds to one hard disk or one side of a magnetooptical disk. A portion of the above-mentioned magnetooptical disk auto-changer employs a system in which a plurality of disks are combined to set a volume having a very large capacity by one logical unit.
The magnetooptical disk auto-changer has a mechanism for moving a disk loaded into a slot in the apparatus by a carrier and setting the same to a disk drive disposed in the apparatus. The disk set to the disk drive is taken out by the carrier when it is no longer needed and moved to the slot. Then, another disk is set to the disk drive. The computer system controls the operation of the disk drive to perform input and output (read/write access) of data to and from the disk which has been set as described above.
The above-mentioned magnetooptical disk auto-changer is able to realize the foregoing information storage system having a very large capacity. However, the magnetooptical disk auto-changer encounters a problem of the performance caused from the movement of the disk in the apparatus. That is, time required to mechanically move the disk is, by about 1000 times, different from time required to perform a process of inputting/outputting data to and from the disk. In particular, when an access to requested data is made by the computer system, application software, which is operated on the system, is inevitably caused to wait for a period of time during which the disk storing required data is moved from the slot to the disk drive and data above is read from the disk. Therefore, the total performance of the system is degraded. In an example case where a connection to a network is established, remote terminal equipment on the network must wait during the foregoing period.
To overcome the problem of the unsatisfactory performance which has arisen due to the movement of the disk, it is effective to reduce the number of disk moving times. Specifically, a method has been suggested in which data, which must be stored in the disk, is previously and collectively managed and data is written at a stretch when a required disk is set to the disk drive so as to reduce the disk change frequencies (U.S. Pat. No. 4,636,946). A system for grouping a plurality of disks to logically use the same as one data storage volume may be combined with a method structured such that data, which is used at an operation, is stored in the same disk or data, to which accesses are made frequently, is concentrically stored in a specific disk so as to reduce the disk interchange frequencies.
As described above, in a case where the magnetooptical disk auto-changer is used to realize an information storage system using a plurality of disks, which can be interchanged, and having a very large capacity, time required for the disk, which is the storage medium, to be moved (interchanged) raises a problem of the deterioration in the performance, which is the reduction in the data access speed. To overcome the foregoing problem, a suggestion has been made in which the above-mentioned method is employed to reduce the disk interchange times. The foregoing method of raising the access speed corresponds to function of optimizing location of data which must be stored in the disk.
However, the optimization of the data location assumes certain data access conditions. In particular, the optimization assumes that, in terms of logical address, randomness of data access is fixed once a volume is created. That is, any conventional technology has not realized a system that dynamically corresponds to change in the data access pattern and manages the data location to optimize performance for each instance of data access pattern.